Ultra-small electronic devices on the nanometer have been the subject of considerable exploratory research. For example, U.S. Pat. No. 5,331,183 describes heterojunctions, diodes, photodiodes, and photovoltaic cells each based on a junction between a conjugated polymer and a fullerene, such as Buckminsterfullerene, C60. The polymer forms a p-type semiconductive donor layer and the fullerene forms an n-type semiconductive acceptor layer. Charge separation in the junction occurs on illumination of the junction.